Interference management Within 3GPP LTE advanced

Transcription

1 Interference management Within 3GPP LTE advanced Konstantinos Dimou, PhD Senior Research Engineer, Wireless Access Networks, Ericsson research

2 Outline Introduction to 3GPP LTE (Advanced) Spectrum Radio Frame Structure OFDM, SC-FDMA Interference Management Goal of interference management in cellular systems Sources of interference within 3GPP LTE Advanced Inter-system Interference Intra-LTE Interference Inter-Cell Interference Inter-Cell Interference Coordination (ICIC)

3 LTE Spectrum Flexibility Operation in differently-sized spectrum allocations From 1.4 MHz to 20 MHz (3GPP Release 8 & 9) system bandwidth Support for paired and unpaired spectrum allocations FDD time Half-duplex FDD (terminal-side only) time TDD time

4 Carrier Aggregation Release 10 Intra- and Inter-Band Carrier Aggregation Contiguous Carrier Aggregation of up to 5 component carriers in each direction One component carrier Non-contiguous intra-band Carrier Aggregation of up to 5 component carriers in each direction One component carrier Inter-band Carrier Aggregation (a.k.a. spectrum aggregation) of up to 5 component carriers in each direction One component carrier

5 Time-domain Structure FDD Uplink and downlink separated in frequency domain One subframe, T subframe = 1 ms One radio frame, T frame = 10 ms UL DL f U L fd L Subframe #0 #1 #2 #3 #4 #5 #6 #7 #8 #9 TDD Uplink and downlink separated in time domain "special subframe" Same numerology etc as FDD economy of scale (special subframe) (special subframe) UL DL f DL/UL UpPTS: Uplink Pilot Time Slot DwPTS: Downlink Pilot Time Slot GP: Guard Period

6 Transmission Scheme Downlink OFDM Parallel transmission on large number of narrowband subcarriers Uplink DFTS-OFDM DFT-precoded OFDM DFT precoder OFDM modulator IFFT Cyclic-prefix insertion DFT IFFT Cyclic-prefix insertion Benefits: Avoid own-cell interference Robust to time dispersion Main drawback Power-amplifier (PA) efficiency Tx signal has single-carrier properties Improved power-amplifier efficiency Improved battery life Reduced PA cost Critical for uplink Equalizer needed Rx Complexity Not critical for uplink

7 Downlink OFDM f 0 Block of M symbols Size-N IFFT M subcarriers CP insertion 0 T u = 1/ f T CP T u T CP-E T u Parallel transmission using a large number of narrowband sub-carriers Multi-carrier transmission Typically implemented with FFT Insertion of cyclic prefix prior to transmission Improved robustness in time-dispersive channels requires CP > delay spread Spectral efficiency loss Configuration, f CP length Symbols per slot Normal 15 khz 4.7 µs 7 Extended 15 khz 16.7 µs khz 33.3 µs 3

8 Physical Resource One frame (10 ms) One subframe (1 ms) One resource element 12 sub-carriers One slot (0.5 ms) T CP T u

9 Uplink DFT-spread OFDM ( SC-FDMA ) Single-carrier uplink transmission efficient power-amplifier operation improved coverage OFDM requires larger back-off than single-carrier DFT-spread OFDM OFDM with DFT precoder to reduce PAR Uplink numerology aligned with downlink numerology Terminal A DFT (M 1 ) 0 IFFT CP insertion M 1 > M 2 Terminal B DFT (M 2 ) 0 IFFT CP insertion

10 Uplink DFT-spread OFDM ( SC-FDMA ) Combined TDMA/FDMA intra-cell orthogonality Scheduled uplink NodeB scheduler controls resource allocation Orthogonal uplink no intra-cell interference Orthogonal uplink relaxed need for fast closed-loop power control Why FDMA component? To support small payloads To handle the case of power limitations Frequency TDMA / FDMA Time

11 Architecture Core network evolved in parallel to LTE EPC Evolved Packet Core Flat architecture, single RAN node, the enodeb Compare HSPA, which has an RNC Internet PSTN Internet Core Network Core Network RNC RNC to other Node Bs to other Node Bs Dedicated channels enodeb UE X2 NodeB UE LTE HSPA

12 Hybrid-ARQ with Soft Combining Same basic structure as HSPA Parallel stop-and-wait processes 8 processes 8 ms roundtrip time To RLC for in-sequence delivery Block 2 Block 3 Block 4 Block 5 Block 1 Hybrid-ARQ protocol Process #7 Process #1 Process #0 Process #2 Process transport block 3 Process transport block 5 Process transport block 2 Process transport block 4 Process transport block 1 Process transport block 1 Process transport block

13 Interaction with RLC Why two transmission mechanisms, RLC and hybrid-arq? Retransmission protocols need feedback Hybrid ARQ [with soft combining] Fast retransmission, feedback every 1 ms interval Frequent feedback need low overhead, single bit Single, uncoded bit errors in feedback (~10-3 ) RLC Reliable feedback (sent in same manner as data) Multi-bit feedback less frequent Hybrid-ARQ and RLC complement each other

14 Multi-antenna transmission techniques Diversity for improved system peformance Beam-forming for improved coverage (less cells to cover a given area) SDMA (or "Multi User MIMO") for improved capacity (more users per cell) Multi-layer transmission ( MIMO ) for higher data rates in a given bandwidth The multi-antenna technique to use depends on what to achieve

15 GOAL OF Interference management

16 Interference within cellular systems Useful signals Interference Serving enhanced Node B (enb) User Equipment (UE) Neighbor enb SINR = I S + N Useful Signal Noise Downlink the UE Interference Uplink the enb Reduce interference so as to increase SINR

17 TYPEs OF Interference

18 TYPES OF INTERFERENCE Inter-system interference From other cellular systems E.g. from WCDMA, IS-95 or from bands belonging to other LTE operators From other types of systems E.g. TV or other broadcasting systems, satelite communications, radars Intra-LTE Interference Inter-cell Interference

19 Inter-system interference Typically of steady nature Exception: interference created by radars transmiting pulses/signals on certain time instants Either on the same frequency "Co-channel interference" or On adjacent frequencies "Adjacent channel interference" Created by hardware imperfections at the transmitter resulting in: - Out of Band/Spurious emissions - Adjacent Channel Leakage Non-perfect filter at the receiver

20 SOlutions to Inter-system interference Adjacent channel interference Receiver blocking Filtering Guard bands Co-channel interference Receiver Desensitization Network Planning Inter-system coordination

21 IntRA LTE interference Other-cell interference Reuse-1 collision => interference collision => interference time time UE 1 enb 1 enb 2 interference UE 2 Empty Resource Block Independent scheduler operation may result in collisions For data: A collision typically leads to some SINR degradation; it does not necessarily mean information loss Collisions more harmful to cell edge users UE 1 UE 2

22 How can a collision be avoided? Radio Resource Management (RRM) Frequency Reuse (FR) Reuse-3 frequency band Coordinated RRM Joint scheduling

23 ICIC for DATA Channels Cost Trade-off analysis "Cost" of a "collision" Fewer user data bits can be carried in one PRB, as the link adaptation needs to select lower modulation order and/or lower coding rate to compensate the lower SINR More HARQ retransmissions may be needed for successful data delivery (due to BER degradation) "Cost" of avoiding a collision Bandwidth restriction: colliding PRBs may need to be banned from use in the neighbor cell or may be used only with restrictions (e.g., with lower power) Delayed scheduling: the scheduling of some UEs (interfering or interfered UEs) may need to be postponed. Ericsson AB 2013 Ericsson External KTH Wireless Seminar In Signals, Sensors, Systems Interference Within 3GPP LTE Advanced

24 What is this result of this trade-off? Downlink: 2X2, Maximum Ratio Combining (MRC) Uplink: 1X1, Single Input Single Output (SISO) Avoiding a collision results in higher loss in radio resource usage than the gain in interference reduction

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